What cheek? None.
Voluntary offloading is a reverse auction. Some people value EUR 150 and might well be quite happy with that. If they do, why not turn a lose-lose issue (for a passenger who does not want to be offloaded as well as for KL) into a win-win?
Presumably someone old enough and with the means to purchase an air ticket and fly between two cities is capable of valuing his own time without having others judge him.
If nobody takes the offer, KL simply increases the offer until it reaches the prescribed involuntary amount and then picks who gets the boot.